Conventional packaging materials for packages of the single-use disposable type generally consist of a plurality of mutually laminated material layers which are together intended to impart to the package the desired mechanical, chemical and other properties. The primary objective of the composition of the packaging materials is to provide the best possible product protection for the product which is to be packed, at the same time the packaging material must be economically viable and easy to produce. In addition, the packaging material must be easy to reform into packages using conventional technology which employs modern, rational packaging machines of the type which form, fill and seal the packages.
Use has long been made within the packaging art of laminated packaging materials (packaging laminates) comprising a configurationally rigid but foldable paper or paperboard layer and outer layers of thermoplastic, preferably polyethylene. In these prior art packaging laminates, the paper or board layer is generally very thick and imparts to the package superior mechanical strength and configurational stability, while the outer thermoplastic layers make the package liquid-tight at the same time as making possible reforming of the packaging laminate into packages by means of so-called heat sealing, in which mutually facing thermoplastic layers are, under the simultaneous application of heat and pressure, brought to surface fusion with one another, for the formation of mechanically strong and liquid-tight sealing joints or seams.
A packaging laminate consisting exclusively of paper or paperboard and polyethylene possesses good mechanical properties and good tightness properties against moisture and liquid, but is almost entirely devoid of tightness properties against gases, for example oxygen gas, which, as a result, may easily penetrate through the packaging walls and come into contact with, and act on, the packed product. In order to avoid such gas contact and gas action on the packed product, it is therefore necessary to supplement the packaging laminate with at least one additional layer of a material which possesses gas barrier properties. One example of such a gas barrier material which is often employed is an aluminium foil (Alifoil) which is applied to the one side of the paper or paperboard layer, between the paper or paperboard layer and one of the outer thermoplastic layers. In itself, an Alifoil is almost absolutely gas-tight, but because of its low level of extensibility, it often happens that the Alifoil cracks in particularly exposed laminate regions during the reforming of the packaging laminate into packages, whereby the desired gas tightness of the package is considerably reduced.
Consequently, for reasons which will readily be appreciated, there has long been a need in the art to find other materials which may replace the conventional Alifoil in the above-described packaging laminate, and one such alternative which already occurs in commercially available packaging laminates is EVOH (an ethyl vinyl alcohol copolymer) which is known, for example, from U.S. Pat. No. Re. 33 376. Other types of gas barrier polymers have also been proposed, such as polyvinylidene chloride, polyamide, polyester, etc., as well as gas barriers of materials other than plastics, for example silicon oxide.
The above-mentioned alternative gas barrier materials possess very good gas barrier properties but, unlike the Alifoil, do not impart to the packaging laminate barrier properties against light, as may many times be desirable when protecting a packed product in order to be able to store the product for a lengthy period of time while retaining its pristine qualities. Examples of products which are sensitive to the action of both oxygen gas and light and which, therefore, require packages possessing both oxygen gas and light barrier properties are milk, juice, wine, cooking oil etc.
The problem inherent in insufficient light tightness in packaging laminates including a gas barrier material of the above-mentioned type, for example a polymer, is solved according to the prior art technology partly by employing unbleached paper or paperboard which gives at least a certain protection against light penetration into the package. This problem is solved more adequately by supplementing the packaging laminate with a metal layer, for example Alifoil, which in addition strengthens the gas barrier properties of the package.